1. Field of Invention
The present invention relates to impedance matching elements for active gain devices and, in particular, to a frequency scalable prematch microwave amplifier with independently characterized active cell and impedance matching elements assembled to optimize input signal power transfer.
2. Description of the Related Art
In the expanding era of high frequency communications applications, such as microwave (RF) amplifiers for satellite communications, matching a high power, low impedance amplifier device to a typical RF input source in order to obtain optimal power transfer is a time consuming and costly procedure. Such impedance matching, or prematching of the amplifier to the source is desirable to provide a broad bandwidth, high power, high gain microwave amplifier.
At high power requirements and high operating frequencies, presently existing microwave amplifiers employ active gain devices such as metal electrode semiconductor field effect transistors (MESFETs), high electron mobility transistors (HEMTs), or bipolar junction transistors to amplify an input signal. In order to match the impedance of the active gain device to the impedance of the input signal at the operating frequency, a passive matching element having a particular impedance value at the operating frequency is usually coupled to the active gain device. For existing amplifiers operating at a relatively high frequency and capable of handling high power requirements, the prematch design procedure is time consuming and costly, increasing the time to market and the overall amplifier cost.
FIG. 1 illustrates an example of a conventional, prior art monolithic microwave integrated circuit (MMIC) metal electrode semiconductor field effect transistor (MESFET) 10 for use in relatively high power, high frequency amplifier applications, such as microwave (RF) communications. The prior art MESFET 10 ordinarily comprises a source electrode 12, a distributed gate electrode 14, and a distributed drain electrode 16, disposed adjacent a semiconductor substrate 18. The gate electrode 14 is shown in dashed lines to indicate that it is provided on a different plane than the drain electrode 16 and the source electrode 12. The substrate 18 is disposed between the source/drain electrode plane and the gate electrode plane, providing a conventional microstrip structure.
The electrode structures of prior art MESFET 10 are illustrated in FIG. 3 only with respect to the metal electrode layers providing connections to respective active distributed semiconductor structures (not shown) within the substrate 18. Accordingly, the prior art MESFET 10, in conventional fashion, is formed in a semiconductor substrate 18 and includes active source, gate, and drain regions (not shown) therein corresponding respectively to the distributed source 12, drain 16, and gate 14 electrodes illustrated in FIG. 3.
More particularly, the distributed drain electrode 16 comprises a plurality of rectangular drain fingers 20 electrically connected by a common drain portion 22. Similarly, the compound source electrode 12 comprises a plurality of rectangular source fingers 24 electrically connected by a common source portion 26. Compound gate electrode 14 comprises a plurality of gate fingers 28 electrically connected to a gate input terminal 30 by a plurality of gate feed connections 32.
The respective drain fingers 20 are each disposed between two adjacent source fingers 24, typically in the same metal layer. The respective drain fingers 20 and source fingers 24 are disposed at a predetermined distance apart. A respective gate finger 28 is disposed substantially in the center between each adjacent source finger 24 and drain finger 20, typically in a different metal layer than that of the drain electrode 16 and the source electrode 12. Hence, the gate electrode 14, drain electrode 16, and source electrode 12 are typically not arranged in a coplanar fashion for a prior art MESFET 10. A width 34 of the prior art MESFET 10 is determined by the length of the source fingers 24.
A length 36 of the prior art MESFET 10 depends, among other factors, on the number of fingers utilized in the distributed device structures. Generally, if the power handling capabilities of the prior art MESFET 10 are to be increased, the number of fingers in each of the source, drain and gate structures is respectively increased which also increases the length 36 of the device.
As the number of fingers in the respective distributed device structures is increased, the impedance of the prior art MESFET 10 adversely decreases. For example, the transistor input impedance can be as low as approximately 1 ohm for multi-watt, high power devices having many gate fingers 28, source fingers 24 and drain fingers 20. In order to compensate for the reduced impedance, an inductor 38 (shown in broken lines) is commonly connected between the source electrode 12 and the far ends of the outermost gate feeds 32, at the edges of the transistor length 36. The inductor 38 is illustrated in broken lines since it connects the gate electrode 14 in a first plane to the source electrode 12 in a second plane, typically through holes (not shown) provided in the semiconductor substrate 18. Providing holes in the substrate 18 typically adds to the cost and complexity of manufacturing and designing a prior art prematch amplifier.
The foregoing arrangement provides a parallel inductance between the gate electrode 14 and the source electrode 12. Source electrode 12 ordinarily acts as the reference or ground node. The parallel inductor 38 is provided having a precharacterized or predetermined inductance value. The prior art MESFET 10 is thereby "prematched" to a predetermined input signal impedance, such as 50 ohms, for a given center frequency or operating frequency. However, the relative cost and effectiveness of "prematching" the prior art MESFET 10 using inductor 38 varies considerably with the frequency of operation, or center frequency, of the signal being applied to the gate input 30 (i.e., the input signal). Therefore, matching a low RF impedance amplifier device, such as the prior art MESFET 10, to a typical 50 ohm input impedance source for a broad bandwidth, high gain microwave amplifier application is a major challenge to circuit designers.
At relatively low center frequencies the physical dimensions of the prior art MESFET 10 are considerably smaller than the wavelength of the input signal to the prematched amplifier. The MESFET 10 and the inductor 38 both act as "lumped" parameter elements at sufficiently low center frequency values. This results in a relatively simple equivalent circuit model for the prior art MESFET 10 at sufficiently low frequencies, and consequently results in a relatively simple impedance prematch design procedure.
For sufficiently low center frequencies, a low frequency equivalent circuit model (not shown) applies. The input impedance of the active gain device, such as a HEMT, bipolar transistor, or the prior art MESFET 10, is generally capacitive. The input impedance is modeled as a "lumped" capacitor connected between the gate 14 and the source 12 of the prior art MESFET 10. The lumped capacitor has a gate-source capacitance (C.sub.GS). At such low frequencies, the inductor 38, or matching element, is also modeled as a single lumped parameter inductive element (L.sub.M). The inductor 38 is placed in parallel with the input gate-source capacitor in the low frequency equivalent model. The resonant frequency .omega..sub.r of the parallel coupling of the gate-source capacitor and the inductor 38 is inversely proportional to the square-root of the product of the capacitance multiplied by the inductance: ##EQU1## The amplifier is prematched by varying the inductance value L.sub.M of the inductor 38 to adjust the value of the resonant frequency .omega..sub.r.
Hence, once the impedance properties of the prior art MESFET 10 are characterized over a range of relatively low center frequencies, the center frequency, operating frequency, or resonant frequency (.omega..sub.r) can be changed, or frequency scaled, by simply changing the inductance value L.sub.M of the parallel inductor 38. The low frequency equivalent circuit will accurately model, or predict, the modified prematched MESFET 10 amplifier. A key aspect to the simplicity of the low frequency prematch procedure is that the inductance 38 is coupled across the input terminals to the MESFET 10 in the low frequency equivalent circuit (not shown), and the inductance 38 is characterized over a range of frequencies independently from the MESFET 10. However, at relatively high frequencies, such as in the microwave (RF) range of operation, the equivalent circuit model becomes much more complex and the amplifier becomes much more difficult to characterize and prematch properly.
As illustrated in FIG. 2, a more accurate high frequency equivalent circuit model 40 of prior art MESFET 10 must account for the "distributed", rather than lumped, transistor parameters stemming from the finite size of the prior art MESFET 10. At relatively high center frequencies, the physical dimensions of the prior art MESFET 10 approach are considerably larger than the wavelength (not shown) of the input signal 52 to the amplifier 60. The MESFET 10 therefore acts as a "distributed" parameter device at sufficiently high center frequency values. The relatively complex high frequency equivalent circuit model 40 for the prior art MESFET 10 consequently results in a relatively expensive and complicated amplifier prematch design procedure.
The high frequency equivalent circuit 40 of prior art MESFET 10 is represented by a plurality of small "lumped" FETS 48 connected by a plurality of lumped gate inductors 50. Hence, the high frequency equivalent circuit 40 includes a plurality of amplification stages 49. The lumped FET models 48 are each connected to a drain node 42 and to a source reference node 44 at respective source and drain leads. The gate inductors 50 couples each successive stage 49 in the equivalent circuit. More significantly, the matching element, or inductor 38 is modeled in parallel with the gate-source input terminals of the very last lumped FET model 48 at the very last stage 49 of the distributed parameter prior art MESFET 10. That is, the inductor 38 is coupled at the output stage of amplifier 60 and is thus "seen through" many lumped stages 49 by the input signal source 62.
An input signal source 62 has a series characteristic impedance element 58 connected to a signal generator 56 operating at a desired center frequency (.omega..sub.r) for the particular amplifier application being designed. The inductor 38 is coupled to prior art MESFET 10 in order to attempt to match the impedance value of the amplifier 60 to approximately the same value as the input signal impedance 58 at the center frequency .omega..sub.c. Such prematching of the amplifier 60 to the input signal impedance element 58 provides optimum power transfer to the amplifier 60, which is highly desirable.
The input signal impedance 58 is typically fifty (50) ohms at the center frequency .omega..sub.c of interest. However, the effect of the inductor 38, as "seen" by the input signal at lead 52 when the input source 62 is connected to an input terminal 54 of the amplifier 60, depends in a non-trivial, complex relationship on the distributed parameters in each active stage 49 of the high frequency distributed equivalent circuit model 40. Thus for high frequencies, the effect of the inductor 38 is frequency dependent and can no longer be characterized over a range of frequencies independently from the characterization of the prior art MESFET 10. Every time either the inductor 38 is changed to a different inductance value L.sub.M, or the MESFET 10 is changed to a different power capability (i.e., is provided with a different length 36), the entire amplifier 60 must be characterized from scratch to check whether an adequate "prematch" has been effected at the center frequency .omega..sub.c of interest.
"Characterizing" a device or element is a non-trivial, time consuming procedure and involves determining the frequency dependent impedance and phase characteristics of the particular device or element. A test source is used to apply a controlled signal to the device or element and instrumentation is used to measure the response to the test source. The test source is applied to the input terminals of the device or element. The test source ordinarily provides a signal having a predetermined center frequency, a predetermined magnitude value, and a phase-reference value. The device or element being characterized must be measured with respect to both its magnitude response (attenuation or gain) and with respect to phase response (time-delay) to the test source signal at the predetermined center frequency.
In the case of the high frequency amplifier 60 discussed supra, the inductor 38 cannot be characterized separately from the prior art MESFET 10. A designer, in order to adequately prematch the amplifier 60, must first select a MESFET 10 having suitable power capabilities (appropriate size). The designer must next estimate an inductance value L.sub.M for inductor 38 which is to be coupled with the MESFET 10 in an attempt to prematch the characteristics of the overall amplifier 60 to the input signal source 62 at the center frequency of interest. The entire amplifier 60 is then characterized to determine whether the actual prematch is within required design tolerances. Many characterizations may be necessary to arrive at a suitable prematch, since the complexity of the equivalent circuit model requires a characterization process that is essentially empirical in nature.
Hence, for a prior art prematch amplifier 60, changing either the desired center frequency or the power capability of an amplifier 60 requires an expensive and time consuming procedure of re-characterizing the amplifier 60 to test the prematch. Accordingly, it is very difficult to provide the prior art amplifier 60 with either power or frequency scalability to a particular application. These typical difficulties adversely increase both the length of the amplifier design cycle and the relative cost of existing prematched prior art microwave amplifiers. For example, at least one additional circuit design iteration is presently required if a prematched transistor, such as prior art MESFET 10, proven for operation at one frequency band of interest (e.g., x-band) is to be operated at a different frequency band of interest (e.g., C-band). The design, wafer fabrication and test cycle for such an additional design cycle iteration extend the time to market by several months and typically increase engineering costs by tens of thousands of dollars.
Therefore, a need exists for resolving the foregoing problems and difficulties presently inherent in providing a prematched microwave amplifier which is easily scalable to operate over a broad range of relatively high power capabilities and over a broad range of relatively high center frequencies.
Accordingly, an object of the present invention is to provide a frequency scalable pre-matched transistor which resolves the foregoing problems, and provides a relatively low cost, high power, short design cycle, prematched microwave amplifier which is easily scalable to operate over a broad range of relatively high power capabilities and over a broad range of relatively high center frequencies.